1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device using a semiconductor wafer grown by a CZ (Czochralski) technique and, especially, a high-impurity concentration semiconductor substrate (wafer).
2. Description of the Related Art
A semiconductor wafer grown by a CZ technique is obtained by cutting a single crystal ingot and polishing it to be a mirror surface. A semiconductor device is often manufactured by applying a predetermined formation process to the semiconductor substrate in this state. According to degree of characteristics required for the device, a semiconductor device may be manufactured using a substrate obtained by stacking epitaxially grown layers.
Ununiformity of an impurity concentration of a semiconductor wafer grown by the CZ technique largely depends on a pulling condition of a single crystal such as nonuniformity of a temperature at the growth interface. For this reason, in order to obtain a uniform concentration, various pulling methods were proposed and performed. However, a single crystal having a completely uniform impurity concentration cannot be obtained as a matter of fact.
When pulling methods are changed to improve uniformity, the resultant substrate becomes expensive. Therefore, this substrate is effective for a semiconductor device having high value added. However, in an inexpensive semiconductor device, a semiconductor substrate having a nonuniform impurity concentration is still used in consideration of profitability.
Especially, in a high-impurity concentration semiconductor wafer, an impurity concentration during pull of a single crystal is difficult to be uniformed, and there is no effective evaluation method. Therefore, a satisfactory method of uniforming an impurity concentration is not obtained at present. For this reason, as a method of uniforming an impurity concentration, a substrate on which an epitaxially grown layer is stacked may be used depending on characteristic ranks. However, in some rank which cannot employ this substrate, a substrate having a nonuniform impurity concentration is used, thereby manufacturing a semiconductor device.
When the impurity concentration in a substrate (wafer) is nonuniformed, variations in characteristics of elements formed in the substrate are increased, thereby causing a lower yield.
This problem will be further described below using a method of forming Zener diodes as an example. In a conventional method, a wafer grown by the CZ technique (such as an As-doped wafer having a resistivity of 4 m.OMEGA..cm or less) is directly used. Oxidation, PEP (photoetching method), CVD (chemical vapor deposition), and diffusion are performed for the wafer to form electrodes, thereby forming a large number of Zener diodes (the size of each chip is about 0.3 .times.0.3 mm.sup.2) in the wafer. In this state, a Zener voltage characteristic of each element is measured to discriminate a non-defective element from a defective element, and color marks are formed on the defective elements to identify them. Then, concentric striae, each formed of defective elements can be observed on the wafer. This is caused by a nonuniform impurity concentration of the semiconductor wafer prior to a predetermined process, since the difference in characteristic among semiconductor elements during the manufactural process is reduced. In the case where it is intended to grow silicon ingots with a high impurity concentration by means of the CZ technique, it may be very difficult to uniform the impurity concentration between the solid phase and the liquid phase. In particular, when an impurity concentration of a melt approaches its solid solution limit, it becomes more difficult to uniform this impurity concentration. Therefore, the silicon ingots may have a distribution of nonuniform impurity concentration. When a wafer formed from such an ingot is employed, the obtained semiconductor elements will have different characteristics, as has been described above. The nonuniformity of the impurity concentration occurs when the single crystal is pulled, as described above. FIG. 3 shows a characteristic distribution of Zener diodes arranged along the diameter of the wafer. The abscissa indicates that position of the wafer which extends along a diameter thereof, and the ordinate indicates a Zener voltage. A curve in FIG. 3 illustrates a relationship between the position of any chip formed in that portion of the wafer which extends the diameter thereof, and the Zener voltage VZ of that chip, and the relationship is illustrated by a continuous curve as a matter of convenience. Thin lines V.sub.1 and V.sub.2 parallel to the abscissa indicate upper and lower limit values of the rated Zener voltage standards of the element. As is apparent from FIG. 3, when a wafer formed by a CZ technique is directly used, the difference in characteristics among the elements, even in a single wafer, is large, and a yield of the elements is low.
As a method of doping an impurity in a semiconductor wafer, there is diffusion, POCl.sub.3 deposition, a CVD method, or the like. Although any one method is selected from these methods, the semiconductor elements formed in the wafer have greatly different characteristics. Therefore, the methods are not suitable for obtaining desired uniformity of the impurity concentration.
As described above, if a predetermined manufactural process is performed to form semiconductor devices on a semiconductor wafer grown by only a CZ technique, the difference in characteristics of the formed elements are increased, and a yield of the elements is decreased due to the nonuniformity of an impurity concentration in the wafer.